Position-measuring devices include mainly rotary encoders or angle-measuring devices, as well as length-measuring devices. Rotary encoders are frequently used as measuring devices for electrical drives, in particular to determine the absolute angular position of motor and drive shafts. Length-measuring devices are employed for measuring linear movements of, for example, a tool carriage in a machine tool. For details regarding the design of such position measuring systems, reference may be made, for example, to the textbook by Alfons Ernst entitled “Digitale Laengen-und Winkelmesstechik” [Digital Longitudinal and Angle Measuring Technology], Moderne Industrie, publishers, (1989).
For purposes of generating an absolute position value, a code carrier has graduation tracks provided thereon which are scanned in a measuring direction by a detector system so as to generate position signals. The position signals, in turn, are processed to yield an absolute position value.
With respect to the graduation tracks, a fundamental distinction is made between incremental and absolute graduation tracks. Incremental graduation tracks are formed by a plurality of graduation elements which are arranged one behind the other at equal intervals and which may be scanned to thereby determine changes in relative position. In contrast, in the case of absolute graduation tracks, the graduation elements are arranged in such a way that an absolute position value can be determined at any desired time. There are known absolute graduation tracks which include several code tracks arranged parallel to each other, so that the absolute position is parallel-coded. Also known are absolute graduation tracks where the absolute position is serial-coded in the form of a chain code. While incremental graduation tracks can in principle achieve higher resolutions, absolute graduation tracks have the advantage of allowing an absolute position value to be determined at any desired time, even immediately after the position-measuring device is turned on.
In order to obtain a high-resolution, absolute position-measuring device, firstly, an incremental graduation track may be provided parallel to an absolute graduation track. Thus, the absolute reference to the absolute graduation track is established, and the high resolution is achieved in conjunction with the incremental graduation track. The absolute graduation track may be parallel-coded or serial-coded, but it may be embodied in such a way that an analog scanning signal is generated during the scanning operation and that the absolute position can be deduced from said analog scanning signal. German Patent Application DE 197 51 853 A1, for example, describes a design for an inductive rotary encoder where the scanning of an inner graduation track yields exactly one period of a sinusoidal wave per revolution. The phase angle of the inner graduation track, in conjunction with an incremental outer graduation track, yields a high-resolution absolute position value.
Another way of implementing an absolute position-measuring device is by means of two or more incremental graduation tracks which are provided parallel to each other and have different graduation periods and which are dimensioned such that the absolute position can be uniquely determined from the phase angles of the scanning signals within the measurement range (one shaft revolution in the case of a rotary encoder). Fundamentals of this technique may be taken, for example, from DE 41 25 865 A1.
It is important, especially with regard to safety aspects, that position and angle values which are determined by the position-measuring device and transmitted to subsequent electronics (e.g., a numerical control system) be reliable; i.e., that even in the event of a technical failure, a usable position or angle value should still be generated in the position-measuring device, or at least that such a failure is either already detected in the position-measuring device and communicated to the numerical control system, or detectable in the numerical control system based on the position or angle values received from the position-measuring device.
In this connection, it is known that two identical position-measuring units having suitable detectors and signal-processing circuits may be provided to generate two independent measured values in the position-measuring device. However, such approaches are very expensive and should therefore be avoided.